1. Field of the Invention
The present invention relates to a phase synchronizer which produces an output signal having a frequency synchronized with an input signal.
2. Description of the Related Art
A disk recording apparatus which uses a magnetic disk as one of various kinds of data recording media comprises a data writer for writing desired data on the magnetic disk and a data reader for reading data written on the magnetic disk. To read data included in a signal to be read from the magnetic disk, the data reader needs a sync signal which has the same frequency as that of the read signal and is in phase with the read signal. To increase the speed of the reading operation, the data reader should quickly produce a stable sync signal.
FIG. 1 shows a block diagram of a conventional phase synchronizer which is installed in a magnetic disk recording apparatus. A phase comparator 1, a frequency comparator 2 and a sync detector 3 receive an input signal f.sub.in read from a magnetic disk. The phase comparator 1, frequency comparator 2 and sync detector 3 also receive an output signal f.sub.out of a voltage controlled oscillator (hereinafter called VCO) 4.
The phase comparator 1 outputs a pulse signal to a selector 5 in accordance with the phase difference between the input signal f.sub.in and the output signal f.sub.out. The frequency comparator 2 outputs a pulse signal to the selector 5 in accordance with the frequency difference between the input signal f.sub.in and the output signal f.sub.out. When the frequency of the input signal f.sub.in coincides with that of the output signal f.sub.out, the sync detector 3 sends out a select signal to the selector 5.
When not receiving the select signal from the sync detector 3, the selector 5 outputs the output signal of the frequency comparator 2 to a charge pump 6. When receiving the select signal from the sync detector 3, the selector 5 outputs the output signal of the phase comparator 1 to the charge pump 6.
The charge pump 6 sends out an analog voltage signal to a loop filter 7 in accordance with the pulse width of the output signal of the phase comparator 1 or the frequency comparator 2. The loop filter 7, which is constituted of a low-pass filter, removes the high-frequency component from the analog voltage signal from the charge pump 6 and outputs a filtered analog voltage signal to the VCO 4.
The VCO 4 outputs the oscillator output signal f.sub.out having a frequency based on the filtered analog voltage output from the loop filter 7.
In the thus constituted phase synchronizer, when the data reading position on, for example, a magnetic disk is changed, the frequency of the input signal f.sub.in varies. As a result, the frequency difference between the input signal f.sub.in and the oscillator output signal f.sub.out of the VCO 4 becomes larger so that the sync detector 3 does not send out the select signal.
Accordingly, the selector 5 outputs the output signal of the frequency comparator 2 to the charge pump 6, which in turn boosts or drops the analog output voltage in accordance with the pulse width of the output signal of the frequency comparator 2.
The loop filter 7 smoothes the analog output voltage of the charge pump 6 and sends the filtered analog voltage signal to the VCO 4. The VCO 4 outputs the oscillator output signal f.sub.out having a frequency based on the filtered analog output voltage from the loop filter 7.
The oscillator output signal f.sub.out of the VCO 4 is fed back to the frequency comparator 2. The feedback in this situation allows the VCO 4 to converge the frequency of the oscillator output signal f.sub.out to the frequency of the input signal f.sub.in.
When the frequency of the oscillator output signal f.sub.out matches the input signal f.sub.in, the sync detector 3 sends the select signal to the selector 5. Consequently, the selector 5 selects the phase comparator 1 over the frequency comparator 2. The selector 5 sends out the output signal of the phase comparator 1 to the charge pump 6. The charge pump 6 boosts or drops the analog output voltage in accordance with the pulse width of the output signal of the phase comparator 1.
The oscillator output signal f.sub.out of the VCO 4 is also fed back to the phase comparator 1. The feedback in this situation operates to reduce the phase difference between the input signal f.sub.in and the oscillator output signal f.sub.out, and the data reading operation is executed based on this oscillator output signal f.sub.out.
In the above-described phase synchronizer, when the output signal of the frequency comparator 2 is provided via the selector 5 to the charge pump 6, there is a frequency difference between the input signal f.sub.in and the output signal f.sub.out.
To improve the synchronizing speed by which the frequency of the output signal f.sub.out converges to the frequency of the input signal f.sub.in in accordance with the analog output voltage signal of the charge pump 6, the VCO 4 should have a high gain. That is, it is desirable that a change in the frequency of the filtered analog output signal of the VCO 4 be greater than a change in the filtered analog output voltage signal of the loop filter 7.
When the output signal of the phase comparator 1 is provided via the selector 5 to the charge pump 6, on the other hand, the frequency of the input signal f.sub.in matches that of the output signal f.sub.out and only a phase difference is present. In this case, to make the phase of the output signal f.sub.out promptly coincide with that of the input signal f.sub.in in accordance with the analog output voltage signal of the charge pump 6 and to stabilize the phase of the output signal f.sub.out, the gain of the VCO 4 should be low. That is, it is desirable that a change in the frequency of the output signal of the VCO 4 be smaller than a change in the filtered analog output voltage of the loop filter 7.
While setting the gain of the VCO 4 to a high gain improves the synchronization speed, the VCO 4 becomes overly sensitive to a slight phase-difference signal output from the phase comparator 1 so that its oscillator output signal f.sub.out does not stabilize. On the other hand, while setting the gain of the VCO 4 a low gain stabilizes the oscillator output signal f.sub.out, the synchronization speed becomes slower. Thus, the conventional phase synchronizer is unable to stabilize the output signal f.sub.out while improving the synchronization speed.